1. Technical Field
A single step process is disclosed to treat a liquid hydrocarbon stream containing sulfones that uses a bundle of vertical hanging fibers inside a shroud to simultaneously carry out the mass transfer and the reaction with alkali metal hydroxide. The sulfur in the sulfone molecules is removed as inorganic sulfite while the rest of the sulfone molecular structure is returned to hydrocarbon. An alkali metal hydroxide solution, such as sodium hydroxide and potassium hydroxide, and a sulfone-containing hydrocarbon stream enter at the top of the shroud and flow down the fibers where mass transfer and conversion of sulfone occurs. A low sulfur hydrocarbon product stream and a sulfite-rich aqueous stream are separately removed from the process. This single step process requires no hydrogen and can be carried out in one vessel thus minimizing space requirements and costs.
2. Description of the Related Art
The presence of sulfur in petroleum fuels is a major environmental problem and regulatory compliance has increasingly forced refiners to produce ultra low sulfur fuels. This is because the sulfur present in fuels is converted when combusted into various sulfur oxides that are then transformed into acids, thus contributing to the formation of hazardous acid rain. These acids also reduce the efficiency and life of catalytic converters in automobiles. Furthermore, sulfur compounds are thought to ultimately increase the particulate content of combustion exhaust gas.
Reducing the sulfur content in hydrocarbon streams, especially in hydrocarbon fuel streams, therefore has become a major objective of environmental legislation worldwide, with major countries imposing very strict limits on the amount of sulfur in diesel fuels. To reduce the sulfur in hydrocarbon streams, refiners typically use catalytic hydrodesulfurizing (“HDS”, a.k.a. “hydrotreating”) processes. In HDS, a hydrocarbon stream that is derived from a petroleum distillation is treated in a reactor that operates at high temperatures and high pressures where sulfur compounds, such as thiophenes, react with hydrogen in the presence of a catalyst (e.g., cobalt and molybdenum sulfides or nickel and molybdenum sulfides supported on alumina). Because of the extreme operating conditions and the consumption of expensive hydrogen, these HDS methods can be costly both in capital investment and operating costs.
Moreover, sometimes conventional HDS or hydrotreating are insufficient to produce a hydrocarbon product in compliance with the current strict sulfur level targets. This is due to the presence of sterically hindered sulfur compounds such as substituted dibenzothiophenes that act as refractory compounds in HDS environments. For example, it is particularly difficult to eliminate traces of sulfur using such catalytic HDS processes when the sulfur is contained in molecules such as dibenzothiophene with alkyl substituents in position 4, or 4 and 6. These species are more prevalent in heavier stocks such as diesel fuel and fuel oil. Attempts to completely convert these species have resulted in increased equipment costs, more frequent catalyst replacements, and degradation of product quality due to side reactions.
One emerging alternative to or an add-on for HDS process is oxidative desulfurization (ODS). In an ODS process, refractory sulfur compounds such as substituted dibenzothphienes in a hydrocarbon fuel stream are oxidized, under mild reaction conditions, into sulfone compounds in the presence of an oxidizing agent and a catalyst. The sulfone compounds are subsequently separated from the hydrocarbon stream. Hydrogen is not needed in ODS processes.
The ODS processes reported in literature vary and include: contact with a mixture of hydrogen peroxide and a carboxylic acid to produce sulfones, which are then degraded by thermal treatment to volatile sulfur compounds; oxidation in the presence of a dilute acid, with the sulfones being extracted using a caustic solution; a combination of the oxidation and thermal treatment steps with hydrodesulfurization; a two-step oxidation and extraction method extracting with a paraffinic hydrocarbon comprising a 3-6 carbon alkane; and various catalytic oxidation processes.
Specifically, techniques for removal of sulfones from oxidized hydrocarbon include extraction, distillation, and adsorption. These separation processes rely on the altered chemical properties such as solubility, volatility, and reactivity of the sulfone compounds when contrasted with the original sulfur compounds.
Liquid-liquid extraction is the conventional option for removing sulfones from oxidized hydrocarbon. Adsorption by solid adsorbent is another option. Both the liquid-liquid and solid-liquid processes result in loss of the entire sulfone molecules to the extracting solvent or the adsorbent. In case of liquid-liquid extraction, the sulfone must be separated from the solvent, usually by distillation, prior to recycling the solvent for further extraction. For solid-liquid adsorption processes, the adsorbent must be disposed of when spent or frequently regenerated due to low adsorption capacity currently achievable. The high operating costs of these multi-step processes have necessitated the development of an alternate technology.
Furthermore, when the sulfones are separated as a liquid, it must be either destroyed in a refinery operation unit such as Fluid Catalytic Cracker and Delayed Coker or find another outlet. Unfortunately, market demands for sulfone in surfactant manufacturing and other industries are insufficient to handle this additional supply.
Therefore, there is a need for a process for removing refractory sulfur from hydrocarbon fuel streams that are more efficient and cost-effective than hydrotreating or HDS. There is also a further need for a process for removing the sulfur while without removing the whole sulfone molecular structure from hydrocarbon fuel stream that has undergone an oxidation process, or so-called “oxidized hydrocarbon fuel”. Both needs are met in our invention by treating an oxidized hydrocarbon fuel stream with an aqueous solution of alkali metal hydroxide to cleave the sulfur atom from the sulfone molecules and by carrying out the cleavage chemistry in a specialty contactor comprising a vertical hanging high surface area fibers, e.g., Merichem Company's Fiber Film® contactor, that is highly efficient for mass transfer between two immiscible phases.